Alloy diffusion coating process



United States Patent 3,467,545 ALLOY DIFFUSION COATING PROCESS Giles F. Carter, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware N0 Drawing. Filed May 29, 1963, Ser. No. 284,010

Int. Cl. C23c 1/10 US. Cl. 117-114 4 Claims ABSTRACT OF THE DISCLOSURE A process of forming an iron-chromium diffusion coating on a ferrous metal article is provided. The process involves immersing the ferrous article in a molten bath containing at least by weight lithium and a source of chromium and which is maintained at a temperature between about 800 C. and the melting point of the article.

The present invention relates to the coating of ferrous rnetal articles by a novel liquid-to-solid transfer process. More particularly, the invention relates to the diffusion of chromium, aluminum, or mixtures of these metals onto ferrous metal articles from a molten bath containing lithium as a transfer agent.

The primary purpose of metal coatings is for surface protection. Clad and coated metals are commonly used materials which have their surfaces protected against corrosion, oxidation, and wear. Most of these metal-to-metal materials commerically available are produced by electroplating, by hot dipping, or by cladding a sheet of one metal to a dissimilar metal. Coatings can also be applied to metal surfaces through diffusion processes. Unfortunately, however, diffusion methods in the .past have been found to be of limited commercial value due to apparatus limitations, inferiority of the coatings obtained, or economic reasons.

It is an object of the present invention to provide a practical method of diffusion-coating ferrous articles with at least one diffusing element selected from the group consisting of chromium and aluminum. The resulting articles have a coating containing one or a combination of the named diffusing elements alloyed with iron. Thus Cr-Fe alloy coatings can be formed on ferrous articles by the invention which exhibit the corrosion resistance characteristics of stainless steel. Also Al-Fe coatings can be formed on ferrous articles which make the surface of the resulting article more resistant to oxidation.

The above and other objects are accomplished in accordance with the present invention by immersing a ferrous metal article in a molten bath containing metallic lithium as a transfer agent and having incorporated therein at least one diffusing element selected from the group consisting of chromium and aluminum; wherein said contacting is carried out at a temperature between about 800 C. and the melting point of said article but more preferably between about 1000 C. and 1200 C.

The diffusion method of the invention is applicable to any ferrous metal article which term as used herein means a metallic substance in which the element iron is present in a predominant amount. Preferably the ferrous metal article will be iron or an alloy which contains at least 50% by weight of iron. In addition to diffusing one or a combination of the diffusion elements onto a ferrous article, the process, if desired, can also be adapted to remove or decrease the amount of either of the named diffusion elements present in a ferrous article treated in order to alter its surface alloy composition.

Although it is not intended to limit the invention to any particular theory of operation, it is believed that the process of diffusing the named elements it best explained 3,467,545 Patented Sept. 16, 1969 ice in terms of an isothermal liquid-to-solid transfer in which the molten lithium acts principally as a solvent and transfer medium to bring the diffusing elements in contact with the solid ferrous metal article accompanied by an isothermal, solid state diffusion process of coating growth. It is found that the greatest thermodynamic tendency for liquid-to-solid transfer to occur is when the molten lithium is saturated With the diffusing element and when the diffusing element is not present in the solid article, though capable of complete solution therein.

Liquid-to-solid transfer results in the incorporation of the diffusing element into the substrate surface. At the high temperatures employed further inward diffusion of the element then causes coating growth. The rate of coating growth is dictated by the well-known laws of solid state diffusion and various for the particular element involved.

The molten metal transfer bath comprises lithium, the diffusing element(s), and any diluent materials which may be present. Lithium may be replaced in part with various diluent materials so as to reduce the amount of lithium required for the diffusion process and to modify the transfer properties of the diffusing elements. Illustrative examples of such diluents are copper, lead, tin, and lithium halide salts. For effective results in the process, lithium must be present in such amount to constitute at least 10% by weight of the bath and preferably above 40% by weight of the bath. The bath may be completely in the molten state with the diffusing element(s) in solution in the lithium. However, in cases where the diffusing element has but a limited solubility such as, for example, chromium, an excess of the solid diffusing element may be present in solid form.

The molten bath for the method of the invention can be prepared in a number of suitable ways. The bath may be formed by heating up a mixture of lithium and the diffusing element(s) together with any desired diluents to process temperature. Alternatively, the diffusing element(s) in selected concentration can be prepared and added to a molten charge of lithium maintained at process temperature. The diffusing element(s) may be added periodically to replenish the bath or added continuously in controlled amounts to facilitate prolonged coating operation. The diffusing element(s) may be added in almost any particle form but generally more uniform coatings are obtained when the element(s) are added as a finely divided powder.

The use of a blanket of inert gas over the molten bath is desirable but not essential since the bath may be operated under carefully controlled conditions in the open atmosphere. It is preferred to agitate the bath during operation by mechanical or some other means but this again is not essential.

The operating temperature of the bath for the process is selected to favorably affect the rate of diffusion of the element(s) consistent with maintaining lithium in the molten state. Generally temperatures less than about 800 C. are not considered practical for metal diffusion because the rate of diffusion is too slow. A preferred operating temperature for the process is in a range of from about 1000 C. up to about 1200 C. Temperatures above 1200 C. may be used with a closed system in order to prevent lithium from distilling off from the bath but in any event the temperature of operation must be maintained below the normal melting point of the solid ferrous metal article treated.

The residence time of the ferrous article in the molten bath for diffusing in the diffusing element(s) influences the thickness of coating obtained and thus may vary widely Depending on the size of the molten bath and the treating time necessary for desired thickness of coating of a particular diffusing element or combination thereof, coiled steel sheet or shaped ferrous metal articles may be passed continuously through the molten bath at a rate to provide the required residence time for a desired coating, or articles may be immersed batch-wise in the molten bath to provide the required residence time for a desired coating and then withdrawn.

No special pretreatment of the ferrous metal articles is required before immersion in the molten bath. It is, of course, desirable, that the surface of the ferrous metal article be clean and for optimum results, it is preferable that the metal article be subjected to conventional degreasing treatment. Nevertheless, it has been Observed that coating formed by the process are not significantly influenced by the presence of scale or thin films of oil on the surface of the base metal.

The ferrous articles treated in accordance with the hereinbefore described method of the invention are termed coated articles although it must be appreciated that the diffusing elements migrate into the solid surface of the ferrous articles and thus alter the characteristics of the articles. For the usual treating times, ranging from approximately 5 minutes to several hours, the coating is characterized by different concentrations of the diffusing elements at its outer surface than are found in the interior.

A better understanding of the invention will be gained from the following illustrative examples of preferred modes of executing the invention. Throughout the examples, the amounts of the various ingredients are given in terms of percent by weight unless otherwise indicated. The concentrations of the diffusing elements reported in the coatings represent a measure of their average concentration in approximately the top 0.3 mil of the coating as determined by X-ray fluorescence. The thicknesses of the coatings reported were determined by microscopic examination of cross sections of the coated articles, after etching by 3% concentrated nitric acid, 97% ethanol in 30-60 seconds. It is to be understood, however, that due to the nature of diffusion coatings, significant concentrations of the diffusing elements can be present in layers of the coated article deeper than the etch test indicates.

Example 1 A molten bath was formed in a carbon steel crucible from 100 g. lithium and one gram powdered chromium. A mild steel coupon was immersed in the bath for 30 minutes at 1100 C. and then withdrawn. An excellent shiney stainless steel coating was formed which upon analysis was found to contain a surface concentration of 28- 30% chromium. This coupon exhibited excellent corrosion resistance to moist $0 a performance similar to that obtained with commmercially known 446 series stainless steel.The coating was stripped in concentrated nitric acid and proved to be a highly ductile coating.

Example 2 A molten bath was formed in a carbon steel crucible from 500 grams lithium and 30 grams aluminum. The bath was agitated and operated under argon. A mild steel (0.06% C) sample treated in this bath for one hour at 105 0 C. produces a coating approximately 2 mils thick. The surface of the sample was not covered by pure aluminum but instead was an alloy of iron-aluminum. This coated article demonstrated excellent resistance to oxidation when exposed to air at temperatures of 1000- 1100 C. The experiment was repeated with a 316 stain- 4 less steel coupon being substituted for the mild steel sample with similar results.

Example 3 A bath containing 500 grams lithium and 20 grams aluminum was heated to 1050 C. After the lithium-aluminum mixture had been stirred for a brief period under argon, 10 grams of chromium powder was added. After stirring for an additional 30 minute period, a mild steel (0.04% C) sample was treated for 30 minutes at the bath temperature maintained at C. The sample was then removed and had a chromium-aluminum-iron alloy coating about 1.5 mil thick. This cooated article demonstrated resistance to oxidation and corrosion far superior to the unmodified base metal treated in the method of the invention.

It is of course, to be appreciated that many well-known treatments can b employed to improve the surface appearance of a coated article if desired. For example, an improved surface finish can be obtained by cold working the base metal to a mirror finish before coating or, alternatively, the surface of the coated article may be cold worked to improve surface appearance. The coated articles also may be subjected to subsequent thermal treatments in order to beneficiate the physical properties such as quenching or annealing.

While other modifications of this inventon which may be employed Within the scope of the invention have not been described, the invention is intended to include all such as may be comprised within the following claims.

I claim:

1. A process for the diffusion coating of a ferrous metal article comprising immersing said article in a molten bath containing at least 10% by weight lithium and a source of chromium, said bath being maintained at a temperature between about 800 C. and the melting point of said article and withdrawing the diffusion coated article from said bath.

2. The process of claim 1 in which the diffusing element is in the form of chromium powder.

3. The process of claim 1 wherein said molten bath contains more than 40% by weight of lithium and said temperature is between 1000 and 1200 C.

4. A method of forming a chromium-containing alloy on a ferrous base metal which comprises providing a molten bath containing lithium as active solvent under an inert gas atmosphere, dissolving chromium in said bath, placing said base metal in said bath, and maintaining said base metal in said bath at a temperature of approximately 15002400 F. until a coating of chromium is obtained thereon.

UNITED STATES PATENTS References Cited 3,086,886 4/1963 Kietfer et a1 117--1 14 X 3,096,205 7/1963 De Guisto 29-1962 2,917,818 12/1959 Thomson 29196.2 3,073,720 1/1963 Mets 117131 X FOREIGN PATENTS 613,224 7/1962 Belgium. 1,312,819 11/1962 France.

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

